1. Field of the Invention
The present invention relates to a dielectric filter for use in a microwave range or a millimeter wave range and also to a duplexer and a communication device using such a dielectric filter.
2. Description of the Related Art
There is an increasing need for a high-capacity and high-speed communication system. To meet such a need, the communication frequency band is being expanded from the microwave band to the millimeter wave band. In particular, the submillimeter wave band is attractive for various applications such as a wireless LAN, a portable video telephone, and a next-generation satellite broadcasting system. As the frequency band expands, a filter is required which is small in size, inexpensive, and suitable for use in a planar circuit. In view of the above, the inventors of the present invention have proposed a "submillimeter wave band-pass filter using a planar-circuit dielectric resonator" (Proceedings of Conference of the Institute of Electronics, Information, and Communications Engineers, 1996, C-121).
The structure of this dielectric filter is shown in an exploded perspective fashion in FIG. 8. In FIG. 8, reference numeral 3 denotes a dielectric plate having electrodes formed on its respective two principal surfaces wherein each electrode is partially removed so as to form non-electrode areas. The non-electrode areas of each electrode are formed at locations corresponding to those of the opposite electrode. In FIG. 8, reference numeral 1 denotes an electrode formed on a surface, on the upper side in FIG. 8, of the dielectric plate 3, and reference numerals 4a, 4b, and 4c denote non-electrode areas. Reference numerals 6 and 7 denote a substrate and a frame, respectively. Both the substrate 6 and the frame 7 are made of ceramic. An electrode is formed on the lower surface of the substrate. An electrode is also formed in the peripheral area 11, outside the frame 7, of the upper surface of the substrate. Furthermore, an electrode is formed on the external side faces of the frame 7. Reference numeral 8 denotes a cover also made of ceramic wherein an electrode is formed on its surface in contact with the electrode 1 and an electrode is also formed on the side faces of the cover. Microstrip lines 9 and 10 serving as probes and also as input/output terminals are formed on the upper surface of the substrate 6.
In the above-described structure, parts of the dielectric plate 3 located between the respective two opposing non-electrode areas serve as TE010-mode dielectric resonators wherein adjacent dielectric resonators are coupled with each other and each resonator is also coupled with the microstrip line 9 or 10.
Because the conventional dielectric filter shown in FIG. 8 has a structure in which the dielectric plate 3 including the dielectric resonators is located between the frame 7 and the cover 8, when the frame 7 is soldered to the substrate 6 to form a single unit, the resultant unit has a warp due to the difference between the linear expansion coefficient of the frame 7 and that of the substrate 6. The dielectric plate 3 having a modulus of elasticity similar to those of the frame 7 and the cover 8 is bonded together with the cover 8 to the upper side of the warped frame 7 via a conductive adhesive. Thus, after these elements are combined together, a stress occurs due to the difference in linear expansion coefficient between the frame 7 and the cover 8 and also due to the warping of the frame 7. The stress can cause the frame 7 or the cover 8 to be separated from the dielectric plate 8. The stress can also cause the dielectric plate 3 to have a crack. Even when the dielectric plate does not encounter separation or crack in normal environments, the stress can cause a reduction in environmental resistance.
Although the rigidity of the frame 7 can be increased by increasing the wall thickness of the frame 7, the result is an increase in the overall size. On the other hand, if the height the frame 7 is increased, the result is an increase in the distance between the probes and the corresponding resonators, which makes it impossible to obtain desired external coupling. As a result it becomes impossible to achieve desired characteristics.
In view of the above, it is an object of the present invention to provide a dielectric filter no longer having the above-described problems. It is another object of the invention to provide a duplexer and a communication device using such a dielectric filter.